To meet customer demand, the commercial printing industry requires the capability of producing colors accurately and consistently. Colors can be defined as a fixed set of colors which may be Pantone® colors, customer logo colors, colors in a customer's proprietary marked patterns, or customer defined colors in the form of an index color table. Consistent color in these areas may determine the difference between success and failure in meeting customer requirements. Customer demands for color accuracy and consistency are typically much tighter for spot colors than for colors within images. A printer model for color control requires the mapping between input variables, C, M, Y and K for a xerographic printer, and the output color coordinates, e.g., L*, a*, and b* in the device independent space. More precisely, it is the derivatives (Jacobian) of the output color coordinates to the input variables that provide the ability to accurately control the spot colors. A Jacobian matrix is used to compute the gain matrix of the spot color controller.
Existing spot color editors often utilize a manual approach to the adjustment of CMYK combinations of spot colors prior to raster image processing (RIPing). For example, the document creator may select a Pantone® color for application in specific areas through a user interface on a printing device or computer monitor, such as that available on the Xerox® DocuSP® Controller. The Pantone-provided CMYK combination for the selected printer is obtained from a look-up table. Prior to RIPing the document in the printer, the operator has the option of entering a spot color editor function and specifying an alternative CMYK combination to achieve the desired color. The document is then RIPed and then printed using the spot color editor combinations where specified, and Pantone combinations otherwise.
This workflow presents various problems, among which is operator error associated with manual adjustments of the CMYK combinations; modifications to the CMYK values may result in more variability from machine to machine. Also, the manually-adjusted CMYK values may require more iteration to achieve the desired color. Due to the manual adjustments it may be difficult for customers to achieve the correct CMYK combinations even after repeated trials.
An automated spot color editor (ASCE) method includes determining appropriate target values for a selected color within a print job. The selected color may be described as being within a color space such as reflectance spectra, L*a*b*, XYZ, LHC, CMYK, RGB, parameters describing color, or a color number. The automated spot color editor modifies or adjusts the selected color by selecting a quality level and a maximum number of iterations. During iterations it computes the CMYK recipe for each spot color until a quality level is reached.
CMYK recipes are computed inside an algorithm module. The basic algorithm requires the use of gamut classification to find the CMYK recipe accurately and in the course of doing so save toner usage. Also, this kind of classification can help to improve the overall attainable gamut by fully utilizing the black separation.
The (ASCE) enhances the accuracy performance of spot colors (arbitrary or customer specified) with inline or offline sensors, using a sensitivity matrix. The matrix is computed for each spot color using a gamut class assignment algorithm and gamut class models (CMY->Lab, MYK->Lab, CYK->Lab, CMK->Lab). Gamut class models are obtained using printer characterization.
A common practice for obtaining the printer model is by printing and measuring test patches over the whole printer gamut. The numerical model with printer characterization data uses a large number of colors (2197 patches×2 replicas×4 gamut classes=17576 colors) for printing and measuring whenever the characterization is called for. Measurement of this kind of patch set is required when the ASCE algorithms each require porting to different machines The measurement process is time consuming and expensive, even when the Inline Spectral sensors (ILS) are used.
There is thus a need for providing an accurate color model without an especially time consuming printer characterization process involving a relatively large number of experimental test patches.
There is also always a need for improved accuracy in the ASCE feature, and in particularly enhanced accuracy in the performance of any spot colors (arbitrary or customer specified) using inline or offline sensors.